High performance PD SOI tunneling-biased MOSFET

ABSTRACT

A new type of partially-depleted SOI MOSFET is described in which a tunneling connection between the gate and the base is introduced. This is achieved by using a gate dielectric whose thickness is below its tunneling threshold. The gate pedestal is made somewhat longer than normal and a region near one end is implanted to be P+ (or N+ in a PMOS device). This allows holes (electrons for PMOS) to tunnel from gate to base. Since the hole current is self limiting, applied voltages greater than 0.7 volts may be used without incurring excessive leakage (as is the case with prior art DTMOS devices). A process for manufacturing the device is also described.

FIELD OF THE INVENTION

The invention relates to the general field of MOSFETs with particular reference to biasing such devices through tunneling.

BACKGROUND OF THE INVENTION

Mobile and portable electronics have advanced rapidly and there is an increasing demand for high performance and low power digital circuits. The main technology approach for reducing power has been power supply scaling. Power supply scaling needs to be accompanied by threshold voltage reduction in order to preserve low V_(t), device performance. Unfortunately, low V_(t), raises sub-threshold leakage.

One solution known to the prior art has been to tie the gate to the substrate so as to operate the device as a dynamic threshold voltage MOSFET (DTMOS). This is illustrated as a plan view in FIG. 1A and schematic diagram FIG. 1B. Seen there is gate pedestal 11 flanked by source and drain 13 and 14 respectively. P+ connector 12 makes a hard connection between the gate 11 and the base 15. In that configuration, the gate input voltage forward biases the substrate/source junction and causes V_(TH) to decrease. But the gate voltage of a DTMOS has to be limited to approximately one diode voltage (−0.7 V at room temperature) to avoid significant junction leakage.

The present invention discloses a solution to this problem which allows an MOS device to operate under power supply voltages larger than 0.7 V.

A routine search of the prior art was performed with the following references of interest being found:

In U.S. Pat. No. 6,261,878 B1, Doyle et al. show a DTMOS process while U.S. Pat. No. 6,118,155 (Voldman) shows another DTMOS process. U.S. Pat. No. 6,268,629 (Noguchi) shows a partially depleted MOS SOI with a tunnel leakage current. U.S. Pat. No. 6,306,691 (Koh) show a DTMOS SOI process.

SUMMARY OF THE INVENTION

It has been an object of at least one embodiment of the present invention to provide an FET device suitable for operation at very low voltage and power.

Another object of at least one embodiment of the present invention has been that said device not be limited to a maximum applied voltage of 0.7 V at room temperature to avoid significant junction leakage.

Still another object of at least one embodiment of the present invention has been to provide a process for manufacturing said device.

These objects have been achieved by eliminating the hard connection between gate and base that is featured in dynamic threshold voltage devices (DTMOS). In its place the present invention introduces a tunneling connection between the gate and the base. This is achieved by using a gate dielectric whose thickness is below its tunneling threshold. The gate pedestal is made somewhat longer than normal and a region near one end is implanted to be P+ in an NMOS device (or N+ in a PMOS device). This allows holes (electrons for PMOS) to tunnel from gate to base. Since the hole current is self limiting, applied voltages greater than 0.7 volts may be used without incurring excessive leakage. A process for manufacturing the device is also described.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plan view of a DTMOSFET (prior art) showing a hard connection from the gate to the base.

FIG. 1B is the schematic circuit equivalent to FIG. 1A.

FIG. 2A is a plan view of a TBMOSFET (present invention) showing a tunneling connection from the gate to the base.

FIG. 2B is the schematic circuit equivalent to FIG. 2A.

FIG. 3 is an approximate cross-section of FIG. 2A showing where hole tunneling between gate and base can occur.

FIG. 4 is the equivalent of FIG. 3 for a PMOS device, showing where electron tunneling between gate and base can occur.

FIG. 5 compares source-drain current vs. gate voltage for DTMOS and TBMOS devices.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Overview

The transistors are fabricated using a partially-depleted 0.1 micron CMOS/SOI technology. The substrates are 8″ SIMOX wafers with a buried oxide thickness of 1500 Å. Partially depleted transistors are processed on a 1900 Å thick silicon film. STI (shallow trench isolation) is used for electrical isolation of the transistors. A polysilicon gate is deposited after thermal growth of gate oxide. One of the main novel features of the invention is the extension of the thin gate oxide layer and p+ polysilicon regions to provide hole tunneling in order to increase the body potential in the on state.

Details

We will disclose the present invention through a description of a process for manufacturing it. In the course of so doing, the structure of the present invention will also become apparent.

Referring now to FIGS. 2A, 2B, and 3, the process of the present invention begins with the provision of silicon body or wafer 23 (FIG. 3) having a first isolation area in the form of buried oxide layer 24 located between about 1,500 and 1,600 Angstroms below top surface 23A of the silicon body. For an NMOS device the silicon wafer would be P-type (FIG. 3), while for a PMOS device it would be N-type (43 in FIG. 4). We will, from here, focus our description on NMOS but it will be understood by those skilled in the art that it can be applied equally well to PMOS devices if the appropriate reversals of conductivity type are made.

A second isolation area in the form of oxide filled trenches, such as 25A and 25B, that extend downwards from top surface 23A as far as buried layer 24 is formed. These trenches are disposed so as to fully enclose a volume of silicon (P-type in FIG. 3 and N-type in FIG. 4), resulting in the formation of P-well 15 in FIG. 3 (N-well 45 in FIG. 4).

Next, dielectric layer 26 is formed on top surface 23A. Our preferred dielectric layer has been thermally grown silicon oxide but the invention will still work if any other dielectric material that is suitable for use as a gate dielectric (silicon nitride for example) is substituted. A key requirement is that the thickness of 26 must be less than the maximum thickness at which tunneling can still be observed (the tunneling threshold of the dielectric layer). Typically, the thickness of the dielectric layer is between about 5 and 100 Angstroms.

This is followed by the deposition of layer 11 (44 in FIG. 4), usually polysilicon, over dielectric layer 26. This polysilicon layer is then patterned to form the gate pedestal shown as 11 in plan view 2A and circuit schematic 2B. The polysilicon gate has a thickness between about 1,300 and 1,500 Angstroms and has a width between about 0.05 and 0.1 microns. Gate 11 extends from a position above STI trench 25A, across the well, to abut STI trench 25B, giving it a length of between about 0.5 and 1 microns. Gate 11 is then used as a hard mask during the removal of all of dielectric layer 26 that is not directly beneath it.

Using a suitable mask, donor ions are implanted in a region that overlaps the gate pedestal 11 on both sides, as seen in FIG. 2A, so as to form source and drain regions (13 and 14 respectively in FIG. 2A) on opposite sides of the gate pedestal. These donor ions are implanted to a concentration between about 10¹⁹ and 10²⁰ ions per cm³. For the PMOS device, acceptor ions would be implanted to a concentration between about 10¹⁹ and 10²⁰ ions per cm³. Additional process steps could be introduced at this stage to produce variations on this general approach (e.g. a lightly doped drain) but these are not germane to the invention.

For a conventional device of the prior art, we would now be at the end of the process. However, a key step is now added to the conventional process. This is the implantation of acceptor ions (through a mask) in region 27 (FIG. 2A) that overlaps end 22 of the gate pedestal (42 in FIG. 4) by between about 0.01 and 1 microns. These acceptor ions are implanted to a concentration between about 10¹⁹ and 10 ²⁰ ions per cm³. For the PMOS device, donor ions would be implanted to a concentration between about 10¹⁹ and 10²⁰ ions per cm³. The presence of the P+ region at the end of gate 11 causes a tunneling connection 21 for holes to be formed. Similarly, the presence of N+ region 42 at the end of gate 44 (FIG. 4) causes a tunneling connection 41 for electrons to be formed.

A comparison between DTMOS (prior art) and TBMOS (present invention) is presented in FIG. 5 which plots source-to-drain current as a function of gate voltage. Curve 51 is for a conventional device (DTMOS) while curve 52 is for a device made as described above (TBMOS). It is obvious that the leakage of DTMOS is significantly larger than that of TBMOS—about three orders of magnitude. This shows that TBMOS can operate at a power supply voltage (V_(dd)) that is greater than 0.7 V. 

What is claimed is:
 1. A process for manufacturing a tunneling-biased NMOSFET comprising: providing a P-type silicon wafer having a top surface and a buried oxide layer located a distance below said top surface, forming oxide filled trenches that extend downwards from said top surface as far as said buried layer, said trenches being disposed so as to fully enclose a volume of P-type silicon, thereby forming a P-well on said top surface, forming a dielectric layer to a thickness that is less than the tunneling threshold of said dielectric layer; depositing a layer of polysilicon on said dielectric layer; patterning said polysilicon layer to form a gate pedestal, having two opposing long sides that extend from a first end over a first oxide filled trench across said P-well to a second end abutting a second oxide filled trench; removing all of said dielectric layer not under said polysilicon gate; through a first mask, implanting donor ions in a first region that overlaps both of said long sides and that does not overlap either of said ends, thereby forming source and drain regions on opposing sides of said gate pedestal; and through a second mask, implanting acceptor ions in a second region that overlaps one of said ends, whereby a tunneling connection is formed between said P-well and that part of said polysilicon gate that is within said second region.
 2. The process described in claim 1 wherein said dielectric layer is selected from the group consisting of all possible gate dielectric materials.
 3. The process described in claim 1 wherein said dielectric layer is formed to a thickness between about 5 and 100 Angstroms.
 4. The process described in claim 1 wherein said polysilicon gate is deposited to a thickness between about 1300 and 1,500 Angstroms and has a width between about 0.05 and 0.1 microns.
 5. The process described in claim 1 wherein said second region overlaps said polysilicon gate by between about 0.01 and 1 microns.
 6. The process described in claim 1 wherein said donor ions are implanted in said first region to a concentration between about 10¹⁹ and 10²⁰ ions per cm³.
 7. The process described in claim 1 wherein said acceptor ions are implanted in said second region to a concentration between about 10¹⁹ and 10²⁰ ions per cm³.
 8. A process for manufacturing a tunneling-biased PMOSFET, comprising: providing a N-type silicon wafer having a top surface and a buried oxide layer located a distance below said top surface; forming oxide filled trenches that extend downwards from said top surface as far as said buried layer, said trenches being disposed so as to fully enclose a volume of N-type silicon, thereby forming an N-well; on said top surface, forming a dielectric layer to a thickness that is less than the tunneling threshold of said dielectric layer; depositing a layer of polysilicon on said dielectric layer; patterning said polysilicon layer to form a gate pedestal, having two opposing long sides that extend from a first end over a first oxide filled trench across said N-well to a second end abutting a second oxide filled trench; removing all of said dielectric layer not under said polysilicon gate; through a first mask, implanting acceptor ions in a first region that overlaps both of said long sides and that does not overlap either of said ends, thereby forming source and drain regions on opposing sides of said gate pedestal; and through a second mask, implanting donor ions in a second region that overlaps one of said ends, whereby a tunneling connection is formed between said N-well and that part of said polysilicon gate that is within said second region.
 9. The process described in claim 8 wherein said dielectric layer is selected from the group consisting of all possible gate dielectric materials.
 10. The process described in claim 8 wherein said dielectric layer is formed to a thickness between about 5 and 100 Angstroms.
 11. The process described in claim 8 wherein said polysilicon gate is deposited to a thickness between about 1300 and 1,500 Angstroms and has a width between about 0.05 and 0.1 microns.
 12. The process described in claim 8 wherein said second region overlaps said polysilicon gate by between about 0.01 and 1 microns.
 13. The process described in claim 8 wherein said acceptor ions are implanted in said first region to a concentration between about 10¹⁹ and 10²⁰ ions per cm³.
 14. The process described in claim 8 wherein said donor ions are implanted in said second region to a concentration between about 10¹⁹ and 10²⁰ ions per cm³. 